The present invention generally relates to multi-component fuel cell power generation systems, and more particularly to a fuel cell system which uses a bipolar interconnection plate between adjacent fuel cell units which is characterized by a high degree of catalytic activity, increased durability, enhanced electrical performance, and other benefits.
Fuel cell systems are known and used for the direct production of electricity from standard fuel materials including fossil fuels, hydrogen, and the like. Fuel cells typically include a porous anode, a porous cathode, and a solid or liquid electrolyte therebetween. Fuel materials are directed along and in contact with the anode of the fuel cell system, while an oxidizing gas (e.g. air or O.sub.2) is allowed to pass along and in contact with the cathode of the system. As a result, the fuel is oxidized, with the oxidizing gas being reduced in order to generate electricity. The electrolyte is designed to allow charge transfer between the anode and cathode. Basic aspects of fuel cell technology including the concepts described above are well known in the art and generally discussed in U.S. Pat. Nos. 4,522,894 to Hwang et al.; 4,555,453 to Appleby; 4,567,117 to Patel et al.; 4,629,537 to Hsu; 4,721,556 to Hsu; 4,950,562 to Yoshida et al.; 4,997,727 to Bossel; 5,034,288 to Bossel; 5,069,987 to Gordon; and 5,110,692 to Farooque et al.
In order to produce fuel cell systems with a high degree of electrical output, stacked arrangements of multiple fuel cell units have been developed. To achieve electrical continuity in stacked systems and in order to separate adjacent fuel cell units, a structure known as a "bipolar interconnection plate" is provided between each fuel cell unit in the stack. Specifically, each bipolar plate in the fuel cell stack is positioned against and in contact with the cathode of one fuel cell unit in the stack and the anode of the next successive fuel cell unit in the stack. As a result, one side of the bipolar plate is positioned against the cathode of one fuel cell unit, while the other side of the bipolar plate is positioned against the anode of another fuel cell unit. The bipolar plates in a stacked fuel cell system are important components which must provide a high degree of electrical conductivity with a sufficient level of thermal shock resistance to ensure efficient operation of the fuel cell system. In addition, each of the selected bipolar plates must have a structure and composition which facilitates the secure attachment thereof to adjacent structures in the fuel cell stack.
Numerous bipolar plate structures have been developed for use in stacked fuel cell systems which incorporate a variety of different materials and designs. For example, U.S. Pat. No. 4,997,727 to Bossel discloses a stacked fuel cell system which uses a plurality of bipolar plate units, with each plate being constructed of a complex nickel alloy containing 15.0% by weight Cr; 2.5% by weight Ti; 0.7% by weight Al; 1.0% by weight Nb; 7.0% by weight Fe; 0.4% by weight Si; 0.5% by weight Mn; and 0.04% by weight C, with the remainder consisting of Ni. U.S. Pat. Nos. 4,629,537 and 4,721,556 to Hsu both disclose electrochemical converter/fuel cell units which use bipolar interconnection plates that are each manufactured from silicon carbide, a selected nickel alloy (e.g. of substantially the same type described above in U.S. Pat. No. 4,997,727 to Bossel), or a platinum alloy. U.S. Pat. No. 4,555,453 to Appleby involves a molten carbonate fuel cell system which uses a plurality of stacked fuel cell units, each being separated by a bipolar plate manufactured from nickel-clad stainless steel.
Other bipolar interconnecting structures are disclosed in U.S. Pat. No. 5,034,288 to Bossel which are manufactured from silicon carbide; an alloy consisting of 80% by weight Ni, 14% by weight Cr, and 6% by weight Fe coated on the oxygen side with a layer of La/Mn perovskite; or an alloy containing the following metals in various amounts: Cr, Al, Ti, Zr, Mn, Si, B, C, and Ni. This alloy is likewise coated on the oxygen side with a layer of SnO.sub.2 doped with Sb.sub.2 O.sub.3 or La/Mn perovskite. U.S. Pat. No. 5,069,987 to Gordon discusses the use of LaCr.sub.0.9 Mg.sub.0.1 O.sub.3 to manufacture bipolar plates in fuel cell systems. Finally, U.S. Pat. No. 4,950,562 to Yoshida et al. discusses a fuel cell system using bipolar interconnecting plates constructed of a heat-resistant alloy material containing cobalt, chromium, nickel, iron, or manganese coated with a composite oxide of the perovskite variety.
Notwithstanding the foregoing developments, stacked fuel cell systems have yet to become commercially viable to a significant degree due to a variety of technical problems including high-cost fabrication methods, a lack of shock resistance (which causes cracking and sealing problems), and high levels of internal resistance. Thus, a need remains for a specialized bipolar plate structure suitable for use in stacked fuel cell systems which avoids these problems. In particular, a need exists for a bipolar interconnection plate which offers the benefits of (1) increased thermal shock resistance; (2) lower processing costs; and (3) increased electrical performance due to higher conductivity levels. As described in greater detail below, the present invention involves a unique bipolar plate structure which satisfies these needs and offers other substantial benefits including but not limited to internal catalytic capabilities which enhance the overall efficiency of the fuel cell stack. In addition, the bipolar plate structure described herein provides beneficial thermal expansion characteristics which enable the completed structure to have a coefficient of thermal expansion which is highly compatible with other components in the fuel cell stack. Thus, the present invention represents an advance in the art of fuel cell design as described in greater detail below.